As semiconductor integrated circuits have become finer and more highly integrated, the individual processes involved in semiconductor manufacturing processes have become more numerous and complicated. As a result, the surfaces of semiconductor devices are not always flat. The presence of step differences on the surfaces of semiconductor devices leads to step breakage of wiring and local increases in resistance, etc., and thus causes wiring interruptions and drops in electrical capacitance. In insulating films, furthermore, such step differences also lead to a deterioration in the withstand voltage and the occurrence of leaks.
Meanwhile, as semiconductor integrated circuits have become finer and more highly integrated, the wavelengths of light sources in semiconductor exposure apparatuses used in photolithography have become shorter, and the numerical aperture or so-called NA of the projection lenses used in such semiconductor exposure apparatuses has become larger. As a result, the focal depth of the projection lenses used in such semiconductor exposure apparatuses has become substantially shallower. In order to deal with such increasing shallowness of the focal depth, there is a demand for even greater planarization of the surfaces of semiconductor devices than that achieved so far.
To describe this in concrete terms, planarization techniques such as that shown in FIG. 11 have become essential in semiconductor manufacturing processes. A semiconductor device 14, an inter-layer insulating film 12 consisting of SiO2, and a metal film 13 consisting of Al are formed on the surface of a wafer 11. FIG. 11(a) shows an example of the planarization of an inter-layer insulating film 12 on the surface of the semiconductor device. FIG. 11(b) shows an example in which a so-called damascene is formed by polishing the metal film 13 on the surface of the semiconductor device.
A chemical mechanical polishing or chemical mechanical planarization (hereafter referred to as “CMP”) technique is widely used as a method for planarizing the surfaces of such semiconductor devices. Currently, the CMP technique is the sole method that can be used to planarize the entire surface of a wafer.
CMP was developed on the basis of wafer mirror surface polishing methods. FIG. 12 is a schematic structural diagram of a polishing (planarization) apparatus used in CMP. This polishing apparatus is constructed from a polishing member 15, a polishing object holding part (this may hereafter be referred to as a “polishing head”) 16, and a polishing agent supply part 18. Furthermore, a wafer 17 which constitutes the object of polishing is attached to the polishing head 16, and the polishing agent supply part 18 supplies a polishing agent (slurry) 19. The polishing member 15 is a member in which a polishing body (this may hereafter be referred to as a “polishing pad”) 21 is pasted to a platen 20.
The wafer 17 is held by the polishing head 16, and is caused to swing while being rotated; furthermore, the wafer 17 is pressed against the polishing body 21 of the polishing member 15 with a specified pressure. The polishing member 15 is also caused to rotate, so that a relative motion is caused to take place between the polishing member 15 and the wafer 17. In this state, a polishing agent 19 is supplied to the surface of the polishing body 21 from the polishing agent supply part 18. This polishing agent 19 diffuses over the surface of the polishing body 21, and enters the space between the polishing body 21 and the wafer 17 as relative motion occurs between the polishing member 15 and wafer 17, so that the polished surface of the wafer 17 is polished. Specifically, favorable polishing is performed as a result of a synergistic effect between mechanical polishing caused by the relative motion of the polishing member 15 and wafer 17 and the chemical action of the polishing agent 19.
FIG. 13 is a schematic diagram which shows another polishing apparatus. In this polishing apparatus, the polishing head 16 is on the lower side, and the wafer 17 is chucked above this polishing head 16. Furthermore, the polishing body 21 has a smaller diameter than the wafer 17, and is pasted to a polishing platen 20 which is installed above. Specifically, the polishing body 21 swings while being caused to rotate together with the polishing platen 20, and is pressed against the wafer 17 with a specified pressure. The polishing head 16 and wafer 17 are also caused to rotate, so that a relative motion is created between the wafer 17 and the polishing body 21. In this state, a polishing agent 19 is supplied to the surface of the wafer 17 from the polishing agent supply part 18; this polishing agent 19 diffuses over the surface of the wafer 17, and enters the space between the polishing body 21 and the wafer 17 as relative motion takes place between the polishing member 15 and wafer 17, so that the polished surface of the wafer 17 is polished.
However, there are many different types of wafers that are polished, and independent polishing conditions (a polishing recipe) must be set according to the respective types of wafers.
For example, in the case of polishing through a multi-layer structure such as Cu damascene, Cu is ordinarily polished by primary polishing, and Ta is polished by secondary polishing. In this case, the uniformity of polishing varies greatly even under the same polishing conditions, as a result of differences in the polishing agent and object of polishing. Accordingly, such a method involves the trouble of setting the polishing conditions anew for each polishing operation. Furthermore, in the case of metal polishing, it is necessary to add an oxidizing agent such as hydrogen peroxide in addition to the polishing agent, and the polishing profile varies according to the amount of this additive even in the case of the same polishing agent. Accordingly, the polishing conditions must be varied in all cases when there is a change in the type of polishing agent, additive or object of polishing.
Polishing conditions include the type of polishing liquid, type of polishing pad, rotational speed of the polishing head and polishing member, swinging speed of the polishing head, and pressing pressure of the polishing head, etc. In the case of the rotational speed of the polishing head and polishing member, swinging speed of the polishing head, and pressing pressure of the polishing head, these conditions are a function of time and a function of the polishing head position.
Conventionally, a method in which the polishing conditions that produce the desired worked shape are discovered by performing test polishing based on trial and error on the basis of experience has been used as a method for setting the polishing conditions in accordance with the type of wafer involved. Numerous wafers are used in this test polishing, and considerable time is required in order to determine the polishing conditions.
Furthermore, even if the type of wafer is specified, and the standard polishing conditions are found, the surface shape prior to polishing of wafers that are actually polished varies according to the production lot. Accordingly, it is necessary to perform further test polishing for each production lot, and to make fine adjustments in the polishing conditions. However, even if such fine adjustments are thus made for each production lot, a problem remains in that variation within lots cannot be handled.
In conventional polishing apparatuses in which the polishing body is larger than the wafer being polished, the following problem arises: namely, the size of the apparatus itself increases as the diameter of the wafer increases. Another drawback is that the replacement of consumed parts that require replacement, such as the polishing pad, is extremely difficult because of its large size. Moreover, in cases where there are indentations and projections in the surface of the wafer prior to polishing due to irregularities in film formation, it is extremely difficult to polish the surface to a flat surface by the appropriate treatment of such indentations and projections. In addition, in the case of wafers in which the initial film shape is an “M” shape or “W” shape as a result of the film formation process, there may be instances in which it is necessary to polish the remaining film to a uniform shape. In the case of conventional polishing apparatuses, it is difficult to meet such demands.
Recently, polishing apparatuses using a polishing body that is smaller than the polished wafer have been developed and used as polishing apparatuses that solve such polishing apparatus problems. Since the polishing body is small in such polishing apparatuses, these polishing apparatuses are advantageous in that the size of the polishing parts in the polishing apparatus can be reduced. Furthermore, in regard to the replacement of consumed parts as well, since the parts have a small size, this replacement work itself is extremely easy.
Furthermore, in the case of such polishing apparatuses using a polishing body that is smaller than the polished wafer, the polishing profile can be freely varied by varying the probability of the polishing body being present on respective portions of the surface of the wafer. Accordingly, cases in which indentations and projections are present in the surface of the wafer prior to polishing can be handled.
However, the fact that such fine adjustments are possible means that the polishing conditions must be determined more precisely. Specifically, the number of polishing conditions is increased, and at the same time, the polishing conditions become more complicated; furthermore, the frequency with which the polishing conditions must be determined is increased, and more wafers and time are required for the determination of a single polishing condition. Moreover, even in cases where fine adjustments are not required, since the polishing body is small, the polishing conditions are still complicated compared to those in a conventional polishing apparatus using a large polishing body.
Specifically, in the case of polishing using a small-diameter pad, variable-speed swinging must be added besides the rotation in order to vary the probability of the pad being present on the wafer surface, and it is necessary to perform load control in which the load is lowered in order to suppress a rise in the polishing rate at the edges of the wafer. Accordingly, as a result of the addition of such control actions, the complexity of the polishing conditions is greatly increased.
A method in which the polishing conditions are determined by simulation has been developed as one solution to the problem of considerable time being required for the determination of the polishing conditions. However, in the polishing process, the polishing body undergoes elastic deformation, and the flow of the polishing agent between the polishing body and the object of polishing is complicated; furthermore, frictional heat is generated during polishing. As a result, it is difficult to express the overall polishing process in terms of numerical equations, so that a numerical model with general applicability has not yet been obtained.